The subject matter disclosed herein relates generally to nuclear medicine imaging systems, and more particularly to single photon emission computed tomography (SPECT) imaging systems and compensating for emission attenuation in SPECT systems, especially in cardiac imaging, using emission data.
Different types of imaging techniques are known and used for medical diagnostic imaging. For example, diagnostic nuclear imaging, such as SPECT imaging, is used to study radionuclide distribution in a subject, such as a patient. Typically, one or more radiopharmaceuticals or radioisotopes are injected into the patient. Gamma camera detector heads, typically including a collimator, are placed adjacent to a surface of the patient to capture and record emitted radiation to thereby acquire image data. Different configurations are known wherein the gamma cameras may remain in a fixed location/orientation (e.g., focused detector modules) relative to an object of interest during a scan or may be rotated about the patient. Image reconstruction techniques, such as backprojection, may then be used to construct images of radiotracer uptake distribution within internal structures of the subject based upon the acquired image or acquired data, such as list data.
While such conventional systems may provide quality images with good diagnostic value, photon attenuation is a major physical factor affecting the quality of reconstructed images in SPECT systems. Such attenuation may occur, for example, due to tissues between the sources of emissions and the system detectors. However, in SPECT imaging, and specifically in cardiology, it is important to obtain an accurate emission image (a three-dimensional 3D map of the radioisotope distribution within the imaged patient) in the presence of attenuation (in large part due to Compton scattered radiation) caused by the patient's body.
In cardiac imaging, photon attenuation accounts for up to 85% loss of emitted photons from the myocardium area. Moreover, data inconsistencies with models used in image reconstruction from a quantitative point of view are also spatially variant (e.g., 70-85% error within myocardium only in some cases). Thus, known reconstruction methods require knowledge of the attenuation map, for example, the 3D model of the patient tissue in areas affecting the radiation arriving at the detector. These methods currently mostly rely on direction transmission measurements that may include a radioactive source that is often ineffective or measurements from an x-ray computed-tomography (CT) system that are costly, as well as can add radiation dose to the patient, additional imaging time, geometrical mis-registration and resolution differences. Models may be used to characterize the attenuation, although actual attenuation may differ substantially. Moreover, because of the high variability of patient sizes and shapes, a “patient standard” can yield a poor reconstruction result.